Define the layer colours in the PCB Editor:

Then Document > Export > PDF... and select the layers you want to print and the Color: option:

You may also have to define the colour/monochrome option in your printer driver.

So, it should be merged layer rather than separated layer selection in the above window?

But I have double sided PCB . and again on the top layer, I can only see the "connectors" without "lines" connecting those connectors.

As you can see, before I export to PDF, the top layer looks like this showing the track lines: (Also I need to remove those ratlines.)

From your image:

it looks like all your tracks (and the MICROALGEA PROJECT text) and the copper area have the same net name so they are all shorted together by the copper area.

• Have you created this pcb just by placing pcb footprints on a pcb canvas and then drawing tracks to connect them up?

To create a PCB without first creating a schematic please see:

https://docs.easyeda.com/en/PCB/Layout-PCB-Without-Schematic/index.html

• Please note however that we do not recommend creating a pcb without first creating a schematic.

Please see the recommended Design Flow:

https://docs.easyeda.com/en/Introduction/Design-Flow-by-Using-EasyEDA/index.html

Please also see:

https://easyeda.com/andyfierman/Welcome_to_EasyEDA-31e1288f882e49e582699b8eb7fe9b1f